The process of fermentation production of a chemical product using a sugar as a raw material is utilized for production of various industrial materials. Examples of sugars that can be industrially used as fermentation feedstocks at present include those derived from edible materials such as sugar cane, starch and sugar beet. However, there is a concern of future shortage of edible materials due to an increase in the world population, which may lead to a rise in the prices. Thus, a process of efficiently producing a sugar liquid from a renewable nonfood resource, that is, cellulose-containing biomass, needs to be constructed.
Cellulose-containing biomass mainly contains lignin, which is an aromatic polymer, and cellulose and hemicellulose, which are polymers of monosaccharides. Examples of methods of producing a sugar liquid using a cellulose-containing biomass as a material include a method in which concentrated sulfuric acid or the like is used for direct hydrolysis of a material cellulose biomass, and a pretreatment-enzymatic saccharification method in which a cellulose-containing biomass is preliminarily subjected to pretreatment such as steaming, pulverization or dilutesulfuric-acid treatment to separate cellulose and hemicellulose from lignin, and hydrolysis is then carried out using a saccharifying enzyme such as cellulase.
In methods of producing a sugar liquid using a cellulose-containing biomass as a material, fermentation inhibitors such as hydroxymethylfurfural (HMF), furfural and vanillin are produced in the process of pretreatment, and these inhibit fermentation of the obtained sugar liquid during production of alcohol or the like by fermentation of the sugar liquid, which has been problematic. Moreover, depending on treatment conditions in the production of a sugar liquid, the sugar concentration in the sugar liquid may be low, and it is therefore necessary to increase several to about 10-fold the sugar concentration before subjecting the sugar liquid to the fermentation step. As a method by which fermentation inhibitors contained in the sugar liquid can be removed and, at the same time, the sugar concentration can be increased, a method comprising filtering a sugar liquid through a nanofiltration membrane and collecting a sugar liquid from the feed side is known (see WO 2009/110374 and WO 2010/067785).
We discovered a problem that, when a sugar liquid is filtered through a nanofiltration membrane, sugar is blocked in the feed side of the nanofiltration membrane, but loss of sugar into the permeate side of the nanofiltration membrane inevitably occurs, resulting in a low yield of the sugar liquid collected from the feed side. In view of this, it could be helpful to provide a method that can reduce, during filtration of a sugar liquid through a nanofiltration membrane, loss of sugar into the permeate side of the nanofiltration membrane.